Photoconductive device and method of producing same



A. Z. CZIPOTT May 1, 1962 PHOTOCONDUCTIVE DEVICE AND METHOD OF PRODUCING SAME Filed Oct. 1, 1956 INVENTOR 4(05 Z. C Z A 077 United States Patent Office 3,032,731 Patented May 1, 1962 3,032,731 PHOTOCONDUCTIVE DEVICE AND METHOD OF PRODUCING SAME Akos Z. Czipott, Alhambra, Califi, assignor, by mesne assignments, to Dresser Industries, Inc., Dallas, Tex., a corporation of Delaware Filed Oct. 1, 1956, Ser. No. 613,177 12 Claims. (Cl. 338-19) arrangement, materials and procedure being such as to.

produce a cell of high sensitivity and low resistivity.

The photoconductivity characteristic of selected speciments of sintered cadmium sulphide is known as the electro-optical art. See,for example, Thomsen and Bube in Review of Scientific Instruments, volume 26, page 664 (1955), citing an improvement over powder-type cadmium.

sulphide photoconductive cells produced, for example, by a, method disclosed by Nicol-Kazan (see Journal of Optical Society of America, volume 45, page 647; 1955). However, the powder-type cells made by any of the known modes were not ohmic, current therefrom varying as the fourth or fifth power of the voltage.

An improved procedure and improved basic material useful in producing superior photoconductive cells of the cadmium sulphide type have been disclosed in a concurrently-filed. application, of Acey L. Floyd and Akos Czipott, Serial No. 613,171, now Patent No. 2,884,507. The material and method there disclosed permits manufacture of cadmium sulphide powder-layer photoconductive cells having high sensitivity and ohmic voltage/current characteristics. The material therein disclosed as preferred for forming the powder layer is principally cadmium sulphide with a minor amount of cupric chloride, but about one percent of cadmium acetate is added to enhance ohmic contact between discrete cadmium sulphide particles, and about 500 parts per million of cadmium chloride (CdCl are added to stabilize the cell characteristics with respect to time. These components are thoroughly mixed while still in fine powder form.

Heretofore, vacuum sublimation of powder mixtures of the type mentioned has been utilized to deposit a film or layer of the material on a non-conductor base, followed, in certain instances, by heat treatment in vacuum environment. The present invention aims to eliminate necessity for operation in vacuum, thus greatly simplifying the procedurev involved in producing photoconductive cells; and the invention further aims to provide a cell of. the type mentioned which. has high sensitivity and low resistivity, and a cell of a protected type. A prime object of the invention, then, is to provide a technique or procedure for making photoconductive cells of high sensitivity and low resistivity at low cost. Another object is to provide an inexpensive. photoconductive cell of high sensitivity and low resistivity- Another object is to pro: vide. an inexpensive photoconductive. cell. An additional object is to provide a photoconductive cell of simple, rugged construction, capable of withstanding considerable mechanical abuse without appreciable. deterioration. Another object is to provide improved methods and procedures comprised in the production of photoconductive cells. A further object is to provide a. procedure for making a substantially continuous deposit, of the layer type, of a finely divided photoconductive material.

The aforestated and other objects and advantages hereinafter made evident are attained by the invention which will hereinafter be explained in connection with a specific exemplary cell and the steps of producing that cell. The exemplary cell and components are illustrated in several stages of manufacture and assembly in the accompanying drawings, in which:

FIGURE 1 is a view of a specific exemplary type of cell base;

FIGURE 2 is a view of the cell base depicted in FIG- URE l with electrical conductor means applied at each end of the base, and a portion of the cell structure broken away to facilitate illustration;

FlGURE 3 is a view of the structure depicted in FIG- URE 2, with electrodes added and with parts of the cell stlucture broken away;

FIGURE 4 is a view similar to FIGURE 3 with a thin coating of potentially photoconductive material applied on an area of the interior surface of the base and in contact with the electrodes thereon;

FIGURE 5 is a view similar to FIGURE 4 showing; diagrammatically the cell structure with the applied ma-;

terial dried;.

FlGURE 6 is a view similar to FIGURES showingthe cell structure after repeated coating and drying;

FIUURE 7 is a view of a sealing cap which provides a cell terminal;

FIGURE 8 is a view of the cell structure depicted in FIGURE 6, with a lower sealing cap afiixed; and

FIGURE 9 is a view of a completed photoconductive cell.

In the practice of the present invention it is preferred to employ as a starting or basic material for the photoconductive layer or film of a photoconductive cell, a mixture of finely divided material of the nature of that hereinbefore indicated and disclosed in the aforementioned concurrently-filed patent application Serial No. 613,171, now Patent No. 2,884,507. The basic photoconductor material, preferably in the form of a fine powder, is made into a slurry or paste by mixing with distilled water, and applied in a uniformly thin layer as by brushing, on a suitable electrically-insulative base. While any shape or form of. electrically-insulative base of suitable material may be employed in the manufacture of cells according to some concepts and features of the invention, the exema plary cell base. selected for illustrated and preferred for cells likelytobe subjected to rough treatment, is of glass or quartz and of tubular configuration, preferably but not necessarily circular in cross-section.

Referring to the drawings and to FIGURE 1 in particular, there is illustrated a cell base in the form of a length of translucent tubing of electrically-insulative material such as, for example, of glass. In the drawings the translucent glass cell body is, for simplicity and clarity of illustration, indicated as being non-transparent; however, the glass or other body material may be transparent as well as translucent. In constructing the illustrative form of photoconductive cell the base is carefully cleaned and electrical conductor or connector means of the nature of those indicated at 12 and 14 in FIGURE 2 may be applied on suitable areas of the base. The, conductor means shown form a part of electrode means for the cell, and while in certain instances such conductor means may be dispensed with, they are, in the exemplary embodiment of a cell, preferably employed and may be applied by any suitable means or method. For example, the conductor means may be applied by painting appropriate.

areas; of thebase with a metallic paint such as a chloroplatinate which, when heated, leaves. a firmly-adherent coating of metal (for example, platinum) on the selected areasof the base. Alternatively, a process such asis disclosed in application of Acey L. Floyd, Serial No. 489,397, filed February 21, 1955, may be used. As indicated in FIGURE 2, a continuous conductive metal coating comprising an inner band-like portion 12a, an end portion 12b and an outer portion 12c, all electrically continuous in forming a conductor means 12, has been applied at one end of base and a similar coating comprising similarly d1sposed portions 14a, 14b and Me has been applied at the base to provide a second conductor means. The functions of these coatings or conductor means as parts of electrode means will presently be made apparent.

Suitably spaced-apart cell electrodes are applied on se lected areas of the base for contact with subsequentlyapplied photoconductive or potentially photoconductive material; and such electrodes may be applied concurrently with the previously described conductor means so as to be integral part thereof, or subsequently and separately applied. As depicted in FIGURE 3, exemplary electrodes and 16 are indicated as applied on relatively welldefined areas of the chosen surface of base 10 and each in contact with a respective one of the conductor means 12 and 14 and forming therewith suitable electrode means for the cell. The electrodes may, as hereinbefore indicated, be applied by painting or printing with a solution of a suitable metal-organic compound, such as a chloroplatinate solution, followed by heating to produce a firmly bonded conductive electrode. Alternatively, and for reasons and with advantages pointed out in concurrently filed copending patent application, Serial No. 613,171, the electrodes may be of iridium applied to the appropriate areas of the base and preferably made firmly adherent thereto by heating. It is evident that the individual electrode means such as 12-15 and 14-16 should be electrically isolated from each other at this stage of cell construction.

Following application of electrodes 15 and 16 to base 10, a coating 17 of a distilled water paste or slurry of a photoconductive or potentially photoconductive material, such as the previously-defined powder mixture, is applied to an area of the cell base between and in contact with electrodes 15 and 16. The slurry may be applied in any suitable manner; for example, by painting or swabbing. The entire interior of the base 10 may, if desired, be thus coated with the slurry or paste; or, alternatively, only an area between and overlapping the electrodes may be coated, as shown. The advantage of a better electrical contact of the coating with the electrodes is secured by applying the coating 17 over an area of each of the electrodes, as indicated by the stippled area in FIGURE 4.

After drying the applied layer or coating of slurry, the cell structure with the coating is preferably also baked or subjected to heat treatment at about 900 C. for a minimum of about five minutes in air or oxygen to sinter the material of the layer, care being taken that the rates of increase and decrease of cell temperature are held as low as possible above practical minimum values to keep to a minimum the number and size of cracks or fissures that develop in the layer of material during drying and heating. It is found that such random cracks may appear in the layer in much the same fashion as cracks are formed in a body of mud as the latter dries. Cracks and fissures of the type mentioned, illustrated at 18 in FIGURE 5, apparently present a multiplicity of barriers to current flow between the electrodes; consequently, the cell at this stage of construction may not be ohmic nor highly photoconductive.

To overcome the eifects of the cracks or fissures 18, the cell structure may be given one or more additional coats or layers of the paste or slurry, each such coating being followed by the previously-described drying, and preferably also the heat treatment, whereby the initiallyformed cracks or fissures are filled to produce a relatively uniform plural-layer deposit of photoconductive material of ohmic character. The coating and heating procedure may in instances have to be repeated to attain the desired degree of photoconductivity; but generally only one additionai coating is required to bring the desirable characteristics of the cell to a level at least equal to those of the basic finely-divided sintered material.

There is illustrated in FIGURE 6 a view showing the interior of a cell structure having such relatively uniform coating of photoconductive material overlapping and in contact with the electrodes 15 and 16. While the cell thus far produced and illustrated with a portion broken away' in FIGURE 6 forms an acceptable photoconductive cell to the ends of which electrical terminals may be easily applied, the electrodes and the photoconductive coating or deposit of the exemplary tubular cell are preferably additionally protected by securing over the ends of the tubular base and in electrical contact with respective electrode-conductor means 1215, and 14-16, metal cap members of suitable configuration and size. Cap members of conductive material of the nature illustrated in FIGURE 7 are preferred. In the latter figure there is illustrated a drawn cup-like cap 19 formed preferably but not necessarily of copper and suitably tinned, which may be fitted over an end of the cell base and firmly secured in place thereon by soldering as by tin or indium soldering. As indicated in FIGURE 9, a cap 19 is afiixed to each end of the base 10 whereby the interior of the cell is hermetically sealed against entry of foreign material and whereby the caps 19 form convenient electrical terminals for the cell.

It is evident that in the particular exemplary cell structure illustrated in FIGURE 9, light may be transmitted through the circumferential wall of the cell base 10 between end caps 19-19, and impinge upon the photoconductive layer enclosed within area 17 therein. It is evident, from the preceding description, that the cell base. need not be of tubular construction but may be in the form of a fiat insulative slip of glass or other suitable material upon which spaced-apart electrodes are provided, with a coating of the powder mixture applied and disposed between and overlapping the electrodes and sintered to produce a photoconductive layer extending between and interconnecting the electrodes. A suitable protective cover, such as a cover plate of glass or quartz, may in that form of cell be applied over the base and coating, and secured to the base with adhesive or other suitable means.

Since the physical form of the cell may be widely varied within the scope of the invention, and since modifications of the inventive concept and procedure will occur to those skilled in the art, it is not desired to be limited to the specific details of the illustrative procedure and exemplary cell herein disclosed, but what is claimed is:

1. A protected photoconductive cell, comprising a translucent tubular base member having interior and exterior surfaces and oppo-ite end openings, two spacedapart electrode means disposed at least in part in the interior of said base member, providing electrical connections for the cell and each extending to opposite ones of said openings, a deposit of photoconductive material disposed in the interior of said base in contact with said electrodes, and electrically conductive closure means sealing the said opposite end openings of said tubular base member and each making electrical contact with one of said electrode means.

2. A protected powder-layer photoconductive cell, comprising a light-pervious tubular base member having opposite end openings with two spaced-apart electrodes on the inner surface of said base member and respective electrical connections to the electrodes extending out of the interior of said base member through said opposite openings, a layer of powdered photoconductive material disposed on the interior of said base member and interconnecting said electrodes, and electrically conductive sealing means sealing the said opposite end openings of the said tubular base member and in electrical contact with said electrodes.

3. A protected photoconductive cell of the powderlayer type, comprising a translucent tubular base member having opposite end openings, two spaced-apart electrode means disposed at least in part in the interior of said base member and providing respective electrical connections each leading outof the interior of the said tubular base members through one of said opposite end openings, a plural-layer deposit of a photoconductive material disposed on the interior surface of said tubular base member and interconnecting said electrode means therein, and electrically conductive end caps sealing each of said opposite end openings of said base member to preclude entry of foreign material into the interior of said cell base and to establish electrical contact with said electrode means.

4. A method of producing a photoconductive cell, comprising sintering a mass of material comprising principally cadmium sulphide, forming a fine powder of the sintered mass, forming a slurry of the powder, providing a cell base with electrodes, applying a layer of the slurry on the base between and interconnecting the electrodes, drying the applied layer, applying a second layer of slurry of the same composition applied to form the first-named layer over the dried layer, and drying the second layer.

5. A method of producing a protected photoconductive cell, comprising providing a translucent tubular base member with two spaced-apart electrodes in the interior thereof, forming a photoconductive deposit in the tubular cell base in contact with the electrodes, and applying conductive end caps on the ends of the cell base each in conductive relation with a respective electrode.

6. A method of producing a photoconductive deposit for a photoconductive cell, comprising forming a slurry containing solids consisting principally of finely divided sintered cadmium sulphide, applying a thin layer of the slurry to a base and drying the applied layer, and applying a second thin layer of slurry of the same composition as that applied to form the first-named layer over the previously applied dried layer in such manner as to fill any cracks and fissures in the first applied layer, and drying the superposed layers to produce a substantially continuous thin photoconductive deposit on the base.

7. A method of producing a substantially ohmic thin deposit of a finely-divided photoconductive material, comprising forming a slurry of the finely-divided photoconductive material, applying a thin layer of the slurry on a base surface, drying the applied layer, baking the layer to render it more adherent to the base surface, applying a second thin layer of slurry of the same composition as that applied to form the first-named layer over the first applied layer to fill cracks and fissures therein, and drying and the baking the superposed layers to form a substantially ohmic thin deposit of the finely-divided photoconductive material.

8. A photoconductive cell including a base, a thin layer-like deposit of photoconductive material thereon comprising a first layer of the photoconductive material which first layer has cracks and fissures therein, and a second layer of the said material superposed over said first layer and including material extending into and filling said cracks and fissures in said first layer.

9. A photoconductive cell comprising: a translucent, insulating, tubular member having opposite end openings; electrically conductive cup-like sealing means adjacent each end of said tubular member, closing each of said openings; and a body of photoconductive material on the interior surface of said tubular member and electrically connected between said sealing means.

10. A photoconductive cell in accordance with claim 9 in which said photoconductive material is composed principally of photoconductive cadmium sulphide.

11. A photoconductive cell comprising: a transparent, insulating, tubular member having opposite end openings; two spaced-apart electrodes carried on the interior surface of said tubular member; a photoconductive deposit on the interior surface of said tubular member, in contact with and interconnecting said electrodes; and electrically conductive end caps on the ends of said tubular member and closing said end openings, each of said end caps being electrically connected to one of said electrodes.

12. A photoconductive cell in accordance with claim 11 in which said photoconductive deposit is composed principally of photoconductive cadmium sulphide.

References Cited in the file of this patent UNITED STATES PATENTS 2,001,672 Carpenter May 14, 1935 2,448,516 Cashman Sept. 7, 1948 2,476,042 Hewlett July 12, 1949 2,567,331 Gaiser et al. Sept. 11, 1951 2,651,700 Gans Sept. 8, 1953 2,659,682 Anderson Nov. 17, 1953 2,765,385 Thomsen Oct. 2, 1956 OTI .ER REFERENCES Indium-Steel Magazine of Nov. 9, 1942, pp. 80, 81, 122-124.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,032 731 May 1 1962 Akos z. C2ipott It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 20 and 21 for speciments read specimens column 3 line 9 for "the base read the other end of the base Signed and sealed this 21st day of Augu t 19620 (SEAL) Iitlest:

ESTON G, JOHNSON DAJUKFLQLADD Attcsting Officer Commissioner of Patents 

